Estimating Gem Weight
Compliments of
The International Gem Society
You cannot weigh a gem in a setting and it
is rarely worth removing. Instead, you can get a close approximation of the weight
by the gem’s volume.
When using this approach, careful
measurements are a necessity. Careful observation is also important. All these
weights are adjusted by the stones proportions and it takes a trained eye to
make those judgments.
Step 1
Measure the gem carefully in every direction. Record all your
measurements and the stones shape, making special note of its proportions. Look
up the specific gravity for the stone you are examining.
Step 2
Apply the proper formula for the gem.
ROUND
Diameter X diameter X depth X SG X .0018
OVAL
Diameter X diameter X depth X SG X .0020
(Diameter = average of diagonal, length and width.)
SQUARE
Diameter X diameter X depth X SG X .0024
(Diameter = average of diagonal, length and width.)
SQUARE
CUSHION
Diameter X diameter X depth X SG X .0018
(Diameter = average of diagonal, length and width.)
SQUARE
step cut
Diameter X diameter X depth X SG X .0023
(Diameter = average of diagonal, length and width.)
SQUARE
with cut corners
Diameter X diameter X depth X SG X .0024
(Diameter = average of diagonal, length and width.)
RECTANGLE
Length X width X depth X SG X .0026
RECTANGLE
step cut
Length X width X depth X SG X .0025
RECTANGLE
with cut corners
Length X width X depth X SG X .0026
RECTANGULAR CUSHION
Diameter X diameter X depth X SG X .0022
(Diameter = Average length and width)
PEAR
Length X width X depth X SG X .0018
HEART
Length X width X depth X SG X .0021
MARQUISE
Length X width X depth X SG X .0017
TRIANGLE
Length X width X depth X SG X .0018
CABOCHONS
The actual range for flat-bottomed cabs is .0023 to .0029. Those
with a bottom bulge range from .0024 to .0030. As a rule, use the following
formulas:
High
domed cabs
Length X width X depth X SG X .0026
Low
domed cabs
Length X width X depth X SG X .0029
Step 3
The above formulas are for
well-proportioned gems with a medium/thin girdle. Make the following
adjustments as required.
GIRDLE
Use an average of the girdle thickness. Do not base your reading
on a bulge at the top of a heart, or the point of a gem.
Thin girdle, subtract 1% to 2%
Slightly thick, add 1% to 2%
Thick, add 3% to 4%
Very thick, add 5% to 6%
Extra thick, add 7% to 10%
PAVILION
BULGE
Slight, add 3% to 5%
Noticeable, add 6% to 8%
Obvious, add 9% to 12%
Extreme, add 13% to 18%
A long culet due to steep pavilion angles can add up to 5%
SHAPE
OUTLINE
On cut corner square and rectangle, wide
corners can decrease weight as much as 5%.
Oval, pear, marquis, and heart, wide wings
or high shoulders can add up to 10%. Occasionally, straight shoulders will
require a deduction of 1% to 5%.
Marquis and sometime pears will have a very
short keel, or none at all. This will reduce the weight by 1% to 3%.
Triangles with straight sides will require a reduction of up to 10%.